1. Field of the Invention
The present invention relates to a radiation image converting material having a light-reflecting layer between a support and a phosphor layer. More particularly, the invention relates to a radiographic intensifying screen, and a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of the Prior Art
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for nondestructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film for enhancing the radiographic speed of the system.
As a method replacing the radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In this method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image converting materials such as the radiographic intensifying screen employed in the conventional radiography and the radiation image storage panel employed in the above-described radiation image recording and reproducing method, consist essentially of a support and a phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (a surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shock.
In the radiographic intensifying screen, the phosphor layer comprises a binder and phosphor particles dispersed therein. When excited with a radiation such as X-rays having passed through an object, the phosphor particles emit light of high luminance (spontaneous emission) in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer of the screen can be exposed sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a ralatively small dose.
In the radiation image storge panel, the phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiographic intensifying screen and radiation image storage panel are desired to have a high radiographic speed (or high sensitivity) and provide an image of high quality (high sharpness, high graininess, ect.). Especially when the object is a human body, the radiographic speed of the screen and the sensitivity of the panel are desired to be increased, even if the level is low, for the purpose of reducing the radiation dose applied to the human body.
For enhancing the radiographic speed of a radiographic intensifying screen or enhancing the sensitivity of a radiation image storage panel, there is known a method of providing a light-reflecting layer between the support and the phosphor layer by depositing a metal such as aluminum, etc. on the surface of the support, laminating a metal foil such as an aluminum foil on the support, or applying a coating dispersion comprising a white pigment (light-reflecting material) dispersed in an appropriate binder onto the support. Examples of the white pigment include titanium dioxide, white lead, zinc sulfide, aluminum oxide, magnesium oxide and alkaline earth metal fluorohalides, as described in Japanese patent provisional publications No. 56(1981)-12600, No. 59(1984)-162500 (corresponding to U.S. application Ser. No. 586,691) and No. 59(1984)-100184 (corresponding to U.S. Pat. No. 4,618,778).
By providing such light-reflecting layer to the intensifying screen, a light emitted by the phosphor of the phosphor layer and advancing towards the support is reflected by the light-reflecting layer without being absorbed in the support or passing through the support so as to be released from the phosphor layer side-surface of the screen. Accordingly, the radiographic film is also exposed to the reflected light. By providing such light-reflecting layer to the panel, a light emitted by the stimulable phosphor of the phosphor layer and advancing towards the support is reflected by said layer and released from the panel surface in the same manner. Accordingly, the reflected light is also detected and converted to electric signals by means of a photosensor.